LeetCode 442 (medium) | Find All Duplicates in an Array | O(n) Time, O(1) Space | Best Explanation
In this video, we solve LeetCode 442 - Find All Duplicates in an Array using an O(n) time and O(1) space approach (excluding output storage).
We explain both the intuition and implementation clearly — step by step, including how to use the index marking trick to track duplicates without using extra space.
Problem Statement:
Given an array nums of length n where each element is in the range [1, n] and appears at most twice, return all numbers that appear exactly twice.
Approach:
Iterate through the array
For each element x, mark nums[abs(x)-1] negative
If already negative → it’s a duplicate
Collect duplicates in the result
Complexity:
Time: O(n)
Space: O(1) (excluding output)
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